Why was p[:] designed to work differently in these two situations?

Question

p = [1,2,3]
print(p) # [1, 2, 3]

q=p[:]  # supposed to do a shallow copy
q[0]=11
print(q) #[11, 2, 3] 
print(p) #[1, 2, 3] 
# above confirms that q is not p, and is a d         


        

Answer 1

del and assignments are designed consistently, they're just not designed the way you expected them to be. del never deletes objects, it deletes names/references (object deletion only ever happens indirectly, it's the refcount/garbage collector that deletes the objects); similarly the assignment operator never copies objects, it's always creating/updating names/references.

The del and assignment operator takes a reference specification (similar to the concept of an lvalue in C, though the details differs). This reference specification is either a variable name (plain identifier), a __setitem__ key (object in square bracket), or __setattr__ name (identifier after dot). This lvalue is not evaluated like an expression, as doing that will make it impossible to assign or delete anything.

Consider the symmetry between:

p[:] = [1, 2, 3]

and

del p[:]

In both cases, p[:] works identically because they are both evaluated as an lvalue. On the other hand, in the following code, p[:] is an expression that is fully evaluated into an object:

q = p[:]